In recent years, liquid crystal displays have been extensively used as a display device for TVs, personal computers, digital cameras, cellular phones, etc. The polarizing plate used in the liquid crystal displays is usually constructed of a polarizing film in the form of a dyed monoaxially stretched polyvinyl alcohol film, and triacetyl cellulose films (TAC films) as protective films attached onto the polarizing film (protective film/polarizing film/protective film). The TAC film has been frequently used as a protective film because of a high transparency and a good optical isotropy thereof. On the other hand, the TAC film tends to be deteriorated in dimensional stability, and further tends to have such a problem that the surface of the TAC film must be previously subjected to saponification treatment with an alkali solution before adhering to the polarizing film.
With the recent tendency toward increase in size and quality of liquid crystal displays, there is an increasing demand for high mechanical strength of these films. In addition, it has been required to prevent deterioration in quality of the films owing to bleed-out of low-molecular weight materials and increased haze when subjected to the alkali treatment. Further, the alkali treatment using a high-concentration alkali solution tends to be undesirable in view of working safety and environmental protection.
To solve these conventional problems, various materials other than the TAC films such as norbornene-based films have been proposed (Patent Documents 1 and 2). However, these other material films using no ordinary resins are expensive owing to high production costs. In consequence, there has been proposed the method of using films formed of polyesters as ordinary resins which are capable of ensuring a good dimensional stability and omitting the alkali treatments causing various problems, and further are advantageous in view of costs.
However, the polyester films by themselves tend to have problems such as deteriorated adhesion property to adhesives used for bonding the polarizing film and the protective film, and a low total light transmittance as compared to that of the TAC films, resulting in a low brightness of a polarizing plate obtained using the polyester films.